This Invention relates to a method of achieving rapid densification of porous articles, and, more particularly, of densifying carbon-carbon composite materials.
Several types of materials are fabricated in a porous initial state and thereafter densified to a fully or nearly fully dense final state. An example of such a material is a carbon-carbon composite material used in aircraft and a spacecraft structures, brakes, and thermal control devices. One such material is composed of carbon fibers woven into a multidimensional array or into planar mats, which are stacked on top of each other in a three-dimensional array termed a preform. This preform containing only fibers is highly porous and has a density of only about 1.1-1.2 grams per cubic centimeter. The preform cannot itself be used in a structural application, because it lacks strength and would disintegrate if too-high a stress were applied.
A composite material having highly desirable properties for use in many elevated temperature applications is formed from this preform by a densification process. Densification is accomplished by introducing a matrix material into the preform, to fill the pores between the fibers.
In current processing technology, carbon is introduced into the preform from an external liquid or gaseous source to make a carbon fiber-carbon matrix composite. A liquid or gaseous source of carbon is contacted to the surface of the preform at an appropriate temperature and pressure. Carbon precursor material is impregnated into the pores from the liquid source or infiltrated into the pores from the gaseous source, or a combination of the two techniques is used. After heating, the carbon deposited between the fibers becomes a carbon matrix that binds the carbon fibers together. The introduction of the carbon gradually increases the density of the material to the range of 1.8-2.2 grams per cubic centimeter, which is suitable for various structural applications. After the carbon is deposited into the pores, the carbon-carbon composite is heated for an extended period of time to graphitize the carbon of the matrix.
The existing impregnation and infiltration techniques are slow, because of surface pore blockage by the carbon deposited from the carbon source. A fraction of the carbon tends to preferentially deposit into the pores near the surface of the preform, reducing the carbon flux into the interior of the preform. Densification tends to occur at the surface first, thereby gradually closing the surface pores and slowing the carbon flux into the interior, so that the interior of the composite material remains porous. It is therefore common practice during densification by these approaches to periodically halt the impregnation or infiltration procedure, machine the densified surface region away, and resume the processing. It has been found that repetition of the carbon deposition and surface machining steps can eventually lead to a dense carbon-carbon composite material. As an example of the time and effort involved, production of a 1/2 inch thick carbon-carbon composite material by this multi-cycle deposition and machining technique typically requires 400-600 hours of infiltration interrupted by 4-5 surface machining operations.
Carbon-carbon composites and other types of materials that are initially porous but can be densified using less costly methods offer great potential for a variety of applications. However, this potential cannot be readily realized at this time due to the time-consuming, expensive densification processing requiring multiple carbon deposition and machining cycles. There is a need for an improved approach that can achieve the required densification in much less time and at a reduced cost, as compared with the present approach. The present invention fulfills this need, and further provides related advantages.